Hackers using a Trojan seized control of over 25,000 Unix servers worldwide to create a potent spam and malware distribution platform.

The attack, dubbed Operation Windigo1, was uncovered by security experts at anti-virus firm ESET, in collaboration with CERT-Bund, the Swedish National Infrastructure for Computing, as well as other agencies.

The backdoor trojan behind the attack was used to plant malicious code on compromised systems as part of an elaborate series of interlinked scams.

The drive-by-download part of the operation was geared towards stealing information. Victims of Operation Windigo included webserver control panel software cPanel and kernel.org.

The Ebury backdoor deployed by the Windigo cybercrime operation has been manually installed by hackers and exploits poor configuration and security controls, rather than a vulnerability in Linux or OpenSSH, according to security researchers.

Researchers at ESET reckon Operation Windigo has been running for nearly three years with only scattered elements of the extensive cybercriminal campaign ever being spotted.

"Windigo has been gathering strength, largely unnoticed by the security community, for over two and a half years, and currently has 10,000 servers under its control," said ESET security researcher Marc-Étienne Léveillé. "Over 35 million spam messages are being sent every day to innocent users' accounts, clogging up inboxes and putting computer systems at risk. Worse still, each day over half a million computers are put at risk of infection, as they visit websites that have been poisoned by web server malware planted by Operation Windigo redirecting to malicious exploit kits and advertisements."

Three in five of the world's websites are running on Linux servers, and ESET researchers are calling on webmasters and system administrators to check their systems to see if they have been compromised. If sysadmins discover their systems are infected, they are advised to wipe affected computers and reinstall the operating system and software. It is essential that fresh passwords and private keys are used, as the existing credentials ought to be considered compromised.

Technologies such as two-factor authentication should be considered to bolster security in the aftermath of the attack.

"We realise that wiping your server and starting again from scratch is tough medicine, but if hackers have stolen or cracked your administrator credentials and had remote access to your servers, you cannot take any risks," explains Léveillé. "Sadly, some of the victims we have been in touch with know that they are infected, but have done nothing to clean up their systems - potentially putting more internet users in the firing line." ®

Bootnote

1Windigo, named after a cannibalistic creature from Algonquian Native American folklore.